In agriculture, and also in other application fields, it is of advantage in several situations to replace--on the Cardan shafts for horsepower transmission--the normal Cardan joint with a homokinetic, wide-angle joint.
As far as the present invention is concerned, the difference between the two mechanisms is to be found in the different number of articulations and in the diverse dimensions. In the simple Cardan joint, two mechanical elements are connected to each other by a cross journal articulation and their axes can thereby take up different relative angular positions with continuity. In the homokinetic, that is constant-velocity universal joint, these same elements are connected with a third intermediate member to which each of the elements is articulated by the relevant cross journal. According to the current nomenclature, the constant-velocity universal joint comprises: an end fork constituting the first mechanical element predisposed for the assembly on a power take-off shaft of a tractor or operating machine; an inner fork constituting the second mechanical element connected to the telescopic part of the Cardan shaft; a central body constituting the third intermediate member of said constant-velocity universal joint.
Because of the central body, the constant-velocity universal joint takes up--if compared with the simple joint--a considerably greater axial dimension. The possibility of realizing angular relative positions between the axis of the outer fork and the one of the inner fork is also greatly increased; in the most technically advanced models, these angles reach a value of at least 80.degree..
As a consequence of the above mentioned situation, the construction of an accident prevention guard to be mounted on the Cardan shaft, exhibits some problems. In the case of the simple joint, the guard must completely cover the joint up to the end of the tube fork. In the case of a constant-velocity universal joint, the guard must also cover the whole length of the central body.
According to a known technique, the guard of the constant-velocity universal joint is obtained by integrating the shaft guard with simple-joints through end extension cones which are suitably prolonged and widened. Such solution, however, exhibits some drawbacks, among which: the fact that, for a certain value of the articulation angle of the constant-velocity universal joint, an interference takes place between the intermediate body and the guard cone, since it is impossible--for obvious reasons of overall dimensions--to increase at will the dimensions of the latter; the fact that with the increasing of the articulation angle, greater and greater portion of the central body projects from the end cone, which portion therefore, results unprotected.